The primary objective of the study was to estimate the risk of accelerated soil acidification following the incorporation of stylo into cropping systems. Soil pH response to similar acid additions was different depending on the initial soil pH, and therefore the monitoring of soil pH in limed and unlimed systems highlighted different processes. In the limed system, where pHCa was above 4.0 in CaCl2, cropping systems induced a significant soil pH drop, well correlated with the export of alkali. The acidification rate of bare soil due to leaching was high and a cowpea–maize rotation did not significantly increase the rate, as the export of alkali was low. The introduction of stylo into the cropping system did not result in accelerated acidification when it was cultivated in rotations as the initial crop. However, when stylo was grown as perennial forage under a cut-and-carry system, with the objective of improving soil porosity, the acidification rate increased significantly.
In unlimed systems, ash alkalinity and crop removal were similar as in the limed systems, but pHCa remained stable at approximately 4.0 independent of the cropping system imposed. This result suggests a threshold value under which protons are consumed without a corresponding decline in pH. Kaolinite, which can be dissolved in this pH range, is probably a key factor in maintaining their stable pH. The disorder in the kaolinite layers observed in the XRD traces and the lack of other minerals susceptible to weathering is consistent with this hypothesis.